This invention relates to a lid to open and close an opening portion of an apparatus, more specifically to a see-through lid which comprises a lid body having an opening formed in the lid body, and a see-through plate covering the opening so that the interior of an apparatus may be observed through the see-through plate.
Conventionally, in a see-through lid of this type, a see-through plate is fixed to the peripheral edge of an opening in a lid body by adhesive bonding or ultrasonic welding or by a combination of these two methods. With the adhesive bonding method, however, the materials of the lid body and the see-through plate are limited to those materials which can be bonded with adhesives. Therefore, this method is not compatible with the use of, for example, polypropylene. Moreover, application of adhesives and joining of members are hard tasks. Also with the ultrasonic welding method, there are restrictions on the materials of the lid body and the see-through plate. In addition, this method requires special work and equipment which increases cost. If the see-through plate is fixed by the adhesive bonding or ultrasonic welding method, furthermore, the adhesion strength cannot easily be identified, so that there is a fear of defective products being produced. If the see-through plate firmly fixed to the lid body is broken or flawed, it is necessary to replace the lid body, not to mention the see-through plate. Moreover, the bonded or welded surfaces may sometimes be visible through the see-through plate spoiling the appearance of the lid.